Conventionally, when manufacturing semiconductor devices, liquid-crystal display devices and the like in a photolithography process, a projection exposure apparatus that transfers a pattern of a photomask or a reticle (hereinafter generally referred to as ‘reticle’) onto an object such as a wafer whose surface is coated with photosensitive agent such as a photoresist or onto a glass plate (hereinafter generally referred to as ‘wafer) via a projection optical system, for example, a projection exposure apparatus of a sequentially moving type such as a reduction projection exposure apparatus by a step-and-repeat method (the so-called stepper), and a scanning projection exposure apparatus by a step-and-scan method (the so-called scanning stepper (also called as a scanner)) has been used.
When performing exposure using this type of projection exposure apparatus, in order to prevent as much as possible exposure defects caused by defocus from occurring, it is necessary to detect the position of a wafer in an optical axis direction of a projection optical system by a focal position detection system (focus detection system) and to conform an area to be exposed (area to which an exposure light is irradiated) on the wafer to a best image-forming plane of the projection optical system (to locate the area to be exposed within a range of depth of focus). For this purpose, it is important to detect a best image-forming plane or a best focus position of a projection optical system with good accuracy and to perform calibration of the foregoing focal position detection system (focus detection system) based on the detection result, that is, adjustment of a detection origin or adjustment of a detection offset.
As a detection method of a best focus position of a projection optical system, a method (exposing method) in which a measurement mark formed on a measurement reticle is transferred via a projection optical system to different positions on a wafer at different positions in an optical axis direction of the projection optical system, a best focus position of the projection optical system is detected based on a formation state of a transferred image of the mark formed on the wafer; and a method (aerial image measurement method) in which actual exposure is not performed, an aerial image measurement unit is arranged on a wafer stage that is placed on an image plane side of a projection optical system, the aerial image measurement unit detects light intensity of a projected image (aerial image) of the foregoing measurement mark, and a best focus position of the projection optical system is detected based on the detection result are known. In this case, the aerial image measurement unit is an all-inclusive term of a unit that is arranged on a wafer stage on which a wafer is mounted, and has a pattern plate on which an aperture pattern having a predetermined shape is formed and a photodetection system that receives a light via the pattern plate.
Conventional detection of a best focus position in an aerial image measurement method has been performed basically in procedures as in the following a. to d. (refer to Patent Documents 1, 2, 3 and the like).
a. In a state where a measurement mark (e.g. to be a mark made up of a line-and-space pattern) is formed, a reticle or the like placed on an object plane of a projection optical system is illuminated by an illumination light and an image of the measurement mark is projected to an image plane by the projection optical system, a wafer stage is moved in a predetermined direction within a two-dimensional plane that is orthogonal to an optical axis of the projection optical system so that a pattern plate is scanned with respect to the projected image, and an aerial image of the measurement mark is measured by a photodetection system receiving a light via the pattern plate during the movement.
b. the aerial image measurement as in the above a. is repeated at a plurality of positions in an optical axis direction of the projection optical system (hereinafter referred to as ‘Z position’ for the sake of convenience).
c. Then, Fourier transform is respectively performed to a light intensity signal waveform of an aerial image at each Z position, and predetermined information such as contrast (an amplitude ratio between a first-order frequency component and a direct current component) that is obtained from the light intensity signal waveform of the aerial image at each Z position is respectively obtained.
d. Then, coordinate positions (Z positions, contrast values) at a plurality of points (e.g. 15 points) that are obtained as a result of the above c. are plotted on an orthogonal coordinate system having Z positions as a horizontal axis and contrast values as a vertical axis, and a best focus position is obtained based on an approximate curve that is obtained by performing the least squares approximation to the plurality of points
However, as can be seen from the forgoing description, in the best focus detection method by the conventional aerial image measurement, an operation of changing the position of the pattern plate (wafer stage) in multiple stages in the optical axis direction of the projection optical system and scanning the wafer stage (pattern plate) with respect to an aerial image at each position is essential, which has required a long period of time for measurement by necessity. As a method to improve such inconvenience, it can be considered that the number of steps described above is reduced, however, detection accuracy of a best focus position is lowered by doing so.
Patent Document 1: the U.S. Patent Application Publication No. 2002/041377
Patent Document 2: Kokai (Japanese Unexamined Patent Application Publication) No. 2002-014005
Patent Document 3: Kokai (Japanese Unexamined Patent Application Publication) No. 2002-198303